X86: Don't drop half of the mask when converting 2-address shufps into 3-address pshufd.

It's debatable whether this transform is useful at all, but for now make sure
we don't generate invalid asm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219084 91177308-0d34-0410-b5e6-96231b3b80d8